1. Field of the Invention
Aspects of the present invention relate to a method for mitigating interference between user equipment in a hierarchical cell structure and to a communication system executing the above method. More particularly, aspects of the present invention relate to a method for mitigating interference which may occur between user equipment located in a macro cell and a femto cell when the femto cell containing a small base station is disposed within the macro cell controlled by a macro base station, and also to such a macro base station and such a small base station executing the above method.
2. Description of the Related Art
FIG. 1 is a schematic view illustrating a communication system with a hierarchical cell structure according to the related art.
Referring to FIG. 1, a communication system includes a macro base station 100 as a normal cellular network, and a plurality of small base stations 200a, 200b, and 200c. Normally, a small base station, such as the plurality of small base stations 200a, 200b, or 200c, is designed as an in-house base station for use in residential or small business environments, hence covers an extremely limited range, and is often referred to as a femto cell or a pico cell. The small base station 200a, 200b, or 200c for controlling such small cells is often referred to as a Home Node B (HNB). The cell coverage area of the small base station 200a, 200b or 200c is much smaller than that of the macro base station 100.
Since the macro base station 100 covers a broader range than the small base station 200a, 200b, or 200c, the macro base station 100 performs frequency allocation using a Frequency Division Duplex (FDD) scheme. The small base station 200a, 200b, or 200c performs frequency allocation using a Time Division Duplex (TDD) scheme. The FDD scheme supports two-way radio communication through two distinct frequency channels. The TDD scheme uses a single frequency channel with time slots to transmit signals in both the downstream and upstream directions.
FIG. 2 is a schematic view illustrating an allocation of frequency bands in a communication system including a macro base station and a small base station according to the related art.
Referring to FIG. 2, the frequency bandwidth allocated to each communication system may be 2 GHz or 2.5 GHz.
In FIG. 2, case (a) shows the allocation of frequency bands for the macro base station and the small base station on the assumption that the frequency bandwidth is 2 GHz. In case (a), two frequency bands 1900˜1920 MHz and 2010˜2025 MHz are allocated to TDD channels used in the small base station 200a, 200b, or 200c. Other frequency bands 1920˜1980 MHz and 2110˜2170 MHz are allocated respectively to uplink (UL) and downlink (DL) channels of FDD used in the macro base station 100.
Case (b) in FIG. 2 shows the allocation of frequency bands for the macro base station and the small base station on the assumption that the frequency bandwidth is 2.5 GHz. In case (b), two frequency bands 2500˜2570 MHz and 2620˜2690 MHz are allocated respectively to uplink (UL) and downlink (DL) channels of FDD used in the macro base station 100. Another frequency band 2570˜2620 MHz is allocated to a TDD channel used in the small base station 200a, 200b or 200c. 
In frequency allocation environments as shown in FIG. 2, a frequency band allocated to FDD channels used in the macro base station may be adjacent to another frequency band allocated to TDD channels used in the small base station. In this case, unfavorable interference may often occur. The following are examples of the types and features of the interference.
1) Macro Base Station 100 and Small Base Station 200a, 200b, or 200c 
Serious interference may occur where the macro base station 100 and a certain one of the small base stations 200a, 200b, and 200c are close to each other. In this case, interference may be mitigated by reducing a transmitting power of the small base station at issue and by increasing a physical distance from the macro base station.
2) Macro Base Station 100 and User Equipment Located in Femto Cell Controlled by Small Base Station 200a, 200b, or 200c 
Interference may occur when certain user equipment (namely, femto user equipment) located in a femto cell controlled by the small base station 200a, 200b, or 200c uses a high transmitting power near the macro base station 100. In this case, interference may be mitigated by maintaining a suitable transmitting power of the user equipment at issue and by regulating the coverage of a related small base station.
3) Small Base Station 200a, 200b, or 200c and User Equipment Located in Macro Cell Controlled by Macro Base Station 100
Interference may occur when certain user equipment (namely, macro user equipment) located in a macro cell controlled by the macro base station 100 uses a high transmitting power near the small base station 200a, 200b, or 200c. In this case, the handover for the user equipment at issue may be performed to one of the small base stations 200a, 200b, or 200c. The loss of performance due to interference may not be heavy since there is a low probability that the macro user equipment will exist near the small base stations.
4) Macro User Equipment and Femto User Equipment
FIG. 3 is a schematic view illustrating an occurrence of interference between user equipment according to the related art.
Referring to FIG. 3, interference may occur as indicated by a dashed arrow when certain macro user equipment 320 located in a macro cell controlled by the macro base station 100 is close to certain femto user equipment 340 located in a femto cell controlled by the small base station 200. For example, interference may occur when the macro user equipment 320 uses a high power near the femto user equipment 340 in order to transmit signals to the macro base station 100. This kind of interference may cause the heaviest loss of performance, so a more efficient method for mitigating interference is required.
In order to mitigate interference between the macro user equipment 320 and the femto user equipment 340, a transmitting power control method, a handover method, and a dynamic channel allocation method may be considered.
The transmitting power control method increases a transmitting power of the small base station 200 in a case where the femto user equipment 340 suffers interference from the macro user equipment 320. However, this method may instead increase interference in the uplink channel of the macro user equipment 320.
The handover method performs handover of the macro user equipment 320 to the small base station 200 or of the femto user equipment 340 to the macro base station 100 when interference occurs. However, considering a handover delay time, a closed access scenario of the small base station, a load balancing between cells, etc., this method may not be sufficient to mitigate interference. Although the handover may be somewhat effective in mitigating interference, this method needs an additional way to mitigate interference.
The dynamic channel allocation method allocates different channels to the macro user equipment 320 and the femto user equipment 340. This method divides sub-frames, which are allocated on the basis of time, into first sub-frames used in the macro user equipment 320 and second sub-frames used in the femto user equipment 340 in order to avoid interference. However, this method may have difficulty in regulating the amount of frames used in the macro user equipment 320 and the femto user equipment 340 respectively.